1. Field of the Invention
The present invention relates to a radiation thermometer for receiving infrared rays emitted from an object for measuring the temperature of the object.
2. Description of the Prior Art
Heretofore, optical systems in radiation thermometers for a low temperature region by receiving a 10.mu.m-band infrared ray have been classified into a dioptic type and a catoptric type. The dioptric type of an optical system involves the problem of an increased cost because it is necessary to produce a lens from a special material capable of passing an infrared ray having a wavelength in a region used in the measurement. A further problem is that due to chromatic aberration it is impossible to completely image infrared rays of all the wavelengths contained in the region. In the above radiation thermometer, therefore, a catoptric type optical system has heretofore been used in many cases.
In a Cassegrain type, one of the typical type of the catoptric type, optical system shown in FIG. 6, a portion of the light beam advancing from an object for measurement toward a concave mirror 2 is shielded by a convex mirror 4 because there is a convex mirror 4 in the optical path from the object to the concave mirror 2, thus making it difficult to obtain a bright optical system. In FIG. 6, a detector 6 is adapted to receive an infrared ray from the object for measurement. A pin-hole plate 8 includes a pin-hole 8a which defines a measuring region. A chopper 10 is driven by a motor M to shutter the incidence of light upon the detector 6 intermittently, and a thermosensitive device 12 for measuring the temperature of the detector 6 itself serves as a basic of measurement.
In another conventional catoptric type optical system shown in FIG. 7 a detector is disposed in place of the convex mirror used in the Cassegrain type optical system. In this case, although there is not provided a convex mirror which shields the Cassegrain type optical system, a chopper 16 is disposed in an optical path extending from the object to a concave mirror 14 and it shields the light beam largely in place of the convex mirror, resulting in the optical system becoming dark. Moreover a complicated structure results because the chopper 16 and a chopper driving motor 18 must be supported in the beam.
The present invention has been accomplished in order to remedy the above-mentioned drawbacks of the conventional optical system, and it is an object thereof to provide a radiation thermometer simple in structure and capable of affording a bright optical system.
It is another object of the present invention to provide an optical meter capable of making an accurate measurement.